1. Field of the Invention
This invention relates generally to an apparatus for detecting high-impedance ground faults, and more specifically, to such an apparatus for detecting high-impedance ground faults on a three wire electrical distribution system.
2. Description of the Prior Art
Electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits that can cause collapse of the power system and serious and expensive apparatus damage. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a fault condition, line currents can increase to several times the normal value, thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line are isolated from the network by circuit breakers actuated by protective relays. The protective relays continuously monitor ac voltages and currents to locate line faults and initiate line isolation via tripping of the appropriate circuit breakers. These faults are located by examining the relationship of the ac voltages and currents.
Many electric utility distribution systems use three-phase four-wire circuits to supply both three-phase and single-phase loads. These circuits are protected against faults by three-phase circuit breakers at the substation, generally equipped with phase-overcurrent and ground-overcurrent relays. Ground-overcurrent relay trip settings must allow for normal circuit load unbalances, coordinate with other protection devices, and prevent tripping on large in-rush currents during cold-load pick-up and normal circuit switching. In some distribution circuits, the unbalanced loads could require a ground relay trip setting as high as one-half the phase relay setting. In other distribution circuits, the ground relay may be omitted entirely because of severe unbalanced conditions that make it no more effective than the phase relays. As a result, high-impedance single-phase-to-ground faults, characterized by low-fault current magnitudes, are not generally detectable by overcurrent protective devices commonly used on distribution circuits because such faults do not produce sufficient current or circuit unbalance to operate the phase-overcurrent or ground-overcurrent protective relays. Such high-impedance faults may result from the breaking of a conductor that hangs free or contacts a high-impedance surface such as a tree, antenna, or asphalt. These faults, although not so destructive to the distribution system, may cause extended service interruption.
The following U.S. patents, all assigned to the assignee of the present invention, disclose novel apparatus for detecting high-impedance ground faults on four-wire electrical power distribution systems: U.S. Pat. Nos. 4,297,740; 4,347,542; 4,357,644; and 4,363,066. The apparatus of U.S. Pat. No. 4,297,740 evaluates the zero sequence current on the distribution system to detect a high-impedance fault. The amount of zero sequence current necessary to trip the protective relay is determined by the phase currents or the positive sequence current. U.S. Pat. No. 4,357,644 discloses an electromechanical apparatus to detect high-impedance faults. The operating quantity for this electromechanical apparatus is the zero sequence current. The restraint quantity is developed using a function of any one of the following signals: any two-phase currents, the positive sequence current minus the zero sequence current, the sum of the three-phase currents, or the positive sequence current. U.S. Pat. No. 4,347,542 discloses an improvement to U.S. Pat. No. 4,357,644 to improve the equality of relay response for a given type of fault for all phase combinations. This improved response is accomplished by modifying the winding arrangement on the magnetic core of the relay. The improved embodiment utilizes the same quantities to form the operating and restraining signals. In U.S. Pat. No. 4,363,066, the operating quantity is a function of the zero sequence current and the restraining quantity is a function of the pre-fault phase current less the zero sequence current. As noted, each of these protective relays are suitable for detecting high-impedance ground faults only on four-wire distribution systems.
For three-wire distribution systems, ground fault relaying is currently accomplished through a single ground source at the substation. For a wye-connected substation transformer, the center of the wye (on the distribution voltage side) is connected to ground either solidly or through an impedance. When a ground fault occurs, a zero sequence current flows through the solidly connected ground wire. In an impedance-grounded system, a zero sequence voltage is developed across the impedance. The zero sequence current or zero sequence voltage is sensed to detect a ground fault. Due to the high zero sequence impedance and the resulting low magnitude of fault current in a three-wire system, it is difficult to detect such a zero sequence current or zero sequence voltage.
If the substation transformer is delta connected, the distribution side is grounded through a zigzag or grounded wye open delta grounding transformer arrangement to detect zero sequence voltage. Here also, the zero sequence voltage for a ground fault is small due to the high zero sequence impedance of the earth ground path. Either of these prior art grounding schemes provides a voltage or current signal when a ground fault occurs on the three-wire system. Although set very sensitively, neither of the schemes can reliably detect high-impedance ground faults, which occur when a phase conductor breaks and contacts the earth or another high-impedance object.